Extreme ultraviolet (EUV) lithography, which is based upon exposure with the portion of the electromagnetic spectrum having a wavelength of 10-15 nanometers, can be used to print features with smaller critical dimension (CD) than other more conventional techniques, such as those utilizing deep ultraviolet (DUV) radiation. For example, an EUV scanner may use 4 imaging mirrors and a Numerical Aperture (NA) of 0.10 to achieve a CD of 50-70 nm with a depth of focus (DOF) of about 1.00 micrometer (um). Alternatively, an EUV scanner may use 6 imaging mirrors and a NA of 0.25 to print a CD of 20-30 nm although the DOF will be reduced to about 0.17 um.
Masking and reflection of EUV radiation brings about a unique set of challenges generally not encountered with DUV radiation. For example, a mask for DUV lithography is transmissive, and layers of materials such as chrome and quartz may be used to effectively mask or transmit, respectively, DUV radiation. Thus, a desired pattern on a DUV mask may be defined by selectively removing an opaque layer, such as chrome, to uncover portions of an underlying transparent substrate, such as quartz. However, virtually all condensed materials absorb at the EUV wavelength, so a mask for EUV lithography is reflective, and the desired pattern on an EUV mask is defined by selectively removing portions of an absorber layer (“EUV mask absorber”) to uncover portions of an underlying mirror coated on a substrate, the mirror, or reflective multilayer (“ML”), generally comprising a number of alternating layers of materials having dissimilar EUV reflectivity constants.
One of the challenges in EUV lithography involves minimizing geometric defects, or the effects thereof, which may be present in a substrate surface underlying the reflective multilayer. Conventional polishing techniques employed upon conventional EUV mask blank substrate materials such as titanium silicate glasses, for example, the glass sold under the trade name ULE™ by Corning Corporation, or glass ceramics, for example, the glass sold under the trade name Zerodur™ by Scott Glass Technologies, generally are capable of reducing defects significantly. Subsequent to application of such polishing techniques, bumps, pits, and scratches sized between about 2 nanometers and about 100 nanometers may still be defined within the substrate surface.
Absent efficient techniques for substantially eliminating such remaining surface defects, their effects may be minimized by adding extra layers to the reflective multilayer. Adding layers to the multilayer, however, complicates the lithography process and increases susceptibility to other masking problems. There is a need for an efficient solution to mitigate the effects of defects defined within EUV substrate surfaces.